Various parts of the eye absorb portions of the incident light which strikes the eyes so that only the unabsorbed or transmitted portions reach the retina. The cornea preferentially absorbs wave lengths up to about 340 nanometers (nm). The natural crystalline lens absorbs most of the ultraviolet wave lengths between 300 and 400 nm. Other parts of the eye absorb portions of the visible spectrum.
For those persons who have had one or both of their natural lenses removed, for example as a result of injury or disease, a condition known as aphakia, UV light is no longer absorbed but is instead transmitted to the retina. Lenses used to replace the natural lens such as IOLs made of polymethyl methacrylate (PMMA) usually contain compounds that function as UV absorbers, preventing transmittance of wave lengths of between about 300-400 nm to the retina.
It has long been recognized that UV absorbers are necessary to prevent degradation in polymers exposed to UV radiation from sunlight or indoor exposure to fluorescent light. U.S. Pat. No. 3,433,539 issued to Skoultchi et al, Feb. 3, 1970, discloses derivatives of 2-(2-hydroxyphenyl) benzotriazoles and their copolymerization with known vinyl monomers for preventing UV degradation.
The value of including UV absorbing compounds in contact lenses and intraocular lenses (IOLs) has also been recognized. In a study done with pigmented rabbits by D. Pitts and M. Lattornal reported in Protection Against UVR Using the Vistacon UV/Bloc Soft Contact Lenses, International Contact Lens Clinic, Vol. 14, No. 1 pp. 22-29 (January 1987) results indicated that UV/Bloc lenses provided protection against ultraviolet radiation damage to the eye, whereas contact lenses without a UV absorber provided essentially no protection against UV damage. Furthermore, studies have shown that use of UV filtering IOLs versus use of IOLs without UV absorbers resulted in a decrease in the incidence of cystoid macular edema, see C. Kraff et al Effect of an Ultraviolet Filtering Intraocular Lens in Cystoid Macular Edema, Ophthalmology, Vol. 92, pp. 366-369. Unlike addition of UV absorbers in polymers in, for example, protective coatings to prevent polymer degradation, UV absorbers in contact lenses and IOLs are desired for protection of the eye rather than protection of the material forming the lens. However, the UV absorbers in the lenses will also help prevent degradation of the lenses.
U.S. Pat. Nos. 4,304,895 and 4,390,676 issued to Loshaek, Dec. 8, 1981 and Dec. 28, 1983 respectively, disclose use of UV absorbers in lenses. In particular UV absorbing compounds which can be polymerized with the monomeric material used to form the lenses are disclosed. Specific UV absorbing compounds disclosed include 2-hydroxy-4-methacryloxy benzophenones and mixtures thereof.
U.S. Pat. No. 4,528,311 issued Jul. 9, 1985 to Beard et al discloses UV light absorbing polymer compositions of 2-hydroxy-5-acryloxy phenyl-2-H-benzotriazoles useful in a variety of organic polymers to provide UV absorption properties. The patent in particular discloses use of the compositions in IOLs and contact lenses.
Hydrogels based on vinyl pyrrolidone containing UV absorbing comonomers are described in Dickstein and Vogl, Functional Polymers XXVI, Co and Terpolymers, Involving Methacrylates, N-Vinyl Pyrrolidone and Polymerizable Ultraviolet Stabilizers and Antioxidants, J. Macromol. Sci-Chem., A 22(4) pp. 387-402 (1985). However, those hydrogels are not suitable for use in IOLs.
Although IOLs made of, for example, PMMA are extensively used, hydrogel IOLs are preferred. They are preferred for several reasons. First, hydrogels are flexible and can be folded and inserted through a smaller incision in the eye during surgery resulting in less trauma to the surgical site. Insertion of PMMA IOLs generally requires an incision in the eye of about 6.5-8 mm, whereas a hydrogel lens requires only about a 3.5-4 mm incision.
Second, hydrogels are considered more biocompatible than conventional IOLs. In this respect if a hydrogel contacts any surface within the eye, for example during implantation, it is less likely to damage that surface, it being hydrophilic like the tissues of the eye. Furthermore, hydrogels are termed YAG laser compatible. This means that when laser procedures for correcting opacification are done, the laser radiation, when it inadvertently strikes the implanted IOL, does less damage to a hydrogel than to a PMMA lens.
Third, hydrogels can be steam sterilized rather than chemically sterilized as is the case with, for example, PMMA lenses. Steam sterilization is desired because it is more effective and eliminates the need for removal of chemicals which can irritate ophthalmic tissue after the lens is implanted.
Hydrogels are desirable for use in lenses, particularly IOLs. However, because of their hydrophilic nature and expanded structure, it has been difficult to incorporate UV absorbing compounds ("UV absorbers") into hydrogels. Prior art UV absorbers are generally hydrophobic and have limited solubility in hydrogels. Due in part to this limited solubility, it has been difficult to copolymerize UV absorbers with hydrogel forming monomers. UV absorbers are preferably copolymerized, rather than physically entrapping the absorber within the hydrogel, to prevent the absorber from being leached out of the UV absorbing hydrogel when the hydrogel is in the aqueous environment of the eye or stored in solution.
In addition to problems of incorporation of UV absorbers into hydrogels, UV absorbers having the required characteristics such as UV absorption between 300-400 nm and hydrolytic stability have been difficult to synthesize.